1. Field of the Invention
The present invention generally relates to image forming apparatuses, such as copy machines, printers, facsimile machines, plotters, and multifunction peripherals (MFP) incorporating multiple image forming functions, such as copying and printing functions. More particularly, the present invention relates to an image forming apparatus having multiple image formation speed modes.
2. Description of the Related Art
An image forming apparatus is known in which a low-speed mode or a high-speed mode can be selected by a user. In the low-speed mode, image quality may be given priority, while in the high-speed mode, speed (productivity) may be given priority. In this type of an image forming apparatus, a drive source, such as a motor, may be connected to an image carrier, such as a photosensitive drum, via a series of drive gears. When the gear ratio of the series of drive gears is fixed, the high-speed mode and the low-speed mode may be switched by varying the number of rotations of the drive source.
In this type of an image forming apparatus, noise may increase in the high-speed mode. The noise during an image formation operation is known to be largely due to the noise level of the gear meshing frequency of drive source gears. The gear meshing frequency is the number of times two gears mesh with each other per second. For example, the gear meshing frequency of a drive source is the number of times a motor gear and a transmission gear mesh with each other per second. Thus, the gear meshing frequency, and hence the noise level, can be reduced by decreasing the number of rotations of the motor in the drive source. Desirably, the gear meshing frequency should be lowered below 100 Hz because the sound of such frequencies is difficult for humans to hear.
The drive source in this type of image forming apparatus may include a so-called FG (frequency-generating) output motor equipped with a frequency generator. Typically, the FG output motor has a pattern of frequency-generating pulse shapes (“FG pattern”) disposed opposite a magnet of a rotating part of the motor. As the motor rotates, electromagnetic induction is caused between the magnet and the FG pattern, thereby producing a pulse current. Based on the pulse current, a feedback control is performed so that the rotating speed of the motor can be controlled (see Japanese Laid-Open Patent Application No. 09-46995, for example). The FG output motors are frequently used as a drive source for image forming apparatuses because of their inexpensive rotation control mechanism.
As mentioned above, the high-speed mode and the low-speed mode may be switched by changing the number of rotations of the drive source when the gear ratio the series of drive gears is fixed. In this case, when the rotation speed of the drive source in the high-speed mode is lowered in order to reduce the noise level of the gear meshing frequency of the drive source gears, the number of rotations for the low-speed mode also decreases because of the fixed gear ratio. As a result, the frequency generator may not be able to produce a sufficient level of pulse signal for the feedback control of the rotation speed of the motor.
Japanese Laid-Open Patent Application No. 2002-089638 discusses a drive apparatus including various motors, a simple planetary gear mechanism as an intermediate speed-reduction mechanism, and various speed-reduction units. In this drive apparatus, the motors and the speed-reduction units can be selectively engaged with the simple planetary gear mechanism on an input and an output end, respectively, in order to reduce vibration and noise.
Japanese Laid-Open Patent Application No. 2007-212806 discusses a rotating drive apparatus including a drive source, a series of gears, and a driven member. The gears are coupled via planetary gears for increasing accuracy of rotation of an output shaft and reducing the size in the shaft axial direction, while allowing the detachment of the driven member from the rotating drive apparatus.